The overall goal of this research is to elucidate the molecular basis of defects in the CTC1-STN1-TEN1 (CST) complex that lead to the human diseases Coats plus and dyskeratosis congenita (DKC). CST is associated with telomeres in a number of organisms including mammals. It has telomeric and non- telomeric functions. Telomeric functions in cancer cells include: replication of the repetitive telomere duplex, telomere length regulation through telomerase inhibition, and C-strand fill in. Little is known about telomeric function in telomerase-deficient somatic cells undergoing telomere shortening. Non-telomeric functions of CST include: promotion of new origin firing following replication fork stalling, prevention of DNA damage, and maintaining genomic stability. Although the STN1/TEN1 components are conserved from yeast to human, CTC1 is not conserved; the protein has proved resistant to structural analysis and little is known about its mode of action. Mutations in CTC1 cause the autosomal recessive, pleiotropic human diseases Coats plus and the short telomere syndrome DKC. We propose that these mutations cause separable effects on telomeric and non-telomeric functions of CTC1 and that the consequences of CTC1 dysfunction depend on telomerase activity. Preliminary results presented in this proposal show the engineering of HCT116 human conditional CTC1 knockout cells that will be a good model to test for the biological functionality of Coats/DKC mutations. Other preliminary results suggest the telomeric and non-telomeric roles of CST are not cell-type specific and that the previously uncharacterized TEN1 has similar functions as CTC1 and STN1. This proposal will further characterize CTC1 structure and function using mutations identified in patients as a starting point. In aim 1, the cellular deficiencies caused by Coats/DKC mutations will be elucidated in the HCT116 knockout background. In aim 2, the consequences of depleting CTC1 in the absence of telomerase will be examined and whether Coats/DKC mutants have the same functionality in this context. In aim 3, CTC1 DNA binding domains and protein interaction partners will be identified to further understand the deficiencies caused by the mutations. Together, these aims will address the biology behind Coats plus and DKC and address the function of a complex critical for maintaining genomic stability.